Field of the Invention
The present invention relates to devices incorporating thin electrochemical cells and their method of manufacture. It particularly relates to devices comprising thin, flexible cells that can be used as lightweight, versatile power sources, for example, as batteries or supercapacitors and especially those based on lithium or lithium-ion based cell chemistries.
Description of the Related Art
Conventional coin or button cells have now been replaced in many of their applications by “soft pack” or pouch cells, which are thinner and more flexible and can attain higher energy densities. Traditional lithium-ion soft pack batteries, for example, comprise active cathode and anode material coated onto aluminium and copper current collector foils, respectively. Alternating layers of these electrode materials are stacked or wound on top of one another to form the functional layers of the battery. This is then contained within an envelope of packaging laminate usually consisting of a layer of aluminium foil with a thermoplastic film bonded to the face intended to face inwards. The thermoplastic film is then heat sealed around the periphery of the cell under vacuum, its function being to hold cell components in intimate contact with one another, the aluminium foil providing an adequate moisture barrier. Especially in the case of flexible cells with few cell layers, this packaging laminate represents a significant proportion of the total mass and also reduces the flexibility of the structure.
Pouch batteries can be based on a variety of different cell chemistries, and a range of electrolyte types can be utilised. Lithium primary batteries and secondary batteries, for example, are commonly made according to a pouch design, and dry polymer, gel and liquid electrolytes have all been incorporated into pouch cells. Examples of lithium primary batteries include lithium/carbon monofluoride (LiCFx) batteries. Unlike primary lithium batteries, lithium ion secondary batteries use as anode an intercalation material (e.g. graphite), with the lithium ions cycling between the anode and cathode during charging and discharging. Examples of lithium ion rechargeable batteries include ones where the active cathode agent is a layered oxide, such as lithium cobalt oxide, ones based on a polyanion, such as lithium iron phosphate, or a spinel, such as lithium manganese oxide.
Similar design considerations apply to supercapacitors (or ultracapacitors), which are also becoming available as soft packaged cells to meet the increasing demands of the portable electronics industry. Such supercapacitors are usually based on carbon-carbon, transition metal oxide or conducting polymer chemistries and include both symmetric and asymmetric cell assemblies.
In the last few years, very thin flat cells have been developed as miniature power sources for space critical devices such as sensors, smart cards and RFID tags. For example, Front Edge Technology Inc manufactures ultra-thin lithium rechargeable batteries for card-type applications which are exceptionally thin at 0.05 mm and which can be bent or twisted without damage. Although they contain lithium, they are composed of solid-state thin films including a non-liquid, ceramic electrolyte, so that there is no risk of toxic liquid electrolyte leaking if the hermetic seal is broken.
Since 2002, Blue Spark Technologies have also developed a range of thin, flexible printed film batteries. For example, US2006/0216586 describes a thin zinc chloride based electrochemical cell in which a dielectric “picture frame” is printed around the perimeter of the active ingredients and is used to seal a top substrate to a bottom substrate so as to form an encased cell, without separate packaging being required. The substrates may be laminated film layers, while the picture frame may be formed from a UV curable adhesive; advantageously, the entire cell may be formed on a printing press. This reference is mainly concerned with arrangements in which an active anode layer and active cathode layer are built up, side by side, on the same substrate layer (co-planar design). A further arrangement in which two active electrode layers overlie one another and are built up from a lower substrate layer is also mentioned (co-facial design of FIG. 21). Where a separate current collector layer is required (the zinc anode does not require a current collector), the reference teaches that this can be applied on a portion of the inner surface of a substrate layer. Contact feed-throughs, which provide an electrical pathway from the cell interior to the exterior, need to be provided and to pass under the frame and to be appropriately sealed. Turning to the laminated film layers of the substrate, these may include a structural layer, an oxide barrier layer and a sealing layer; such layers may include metallised or foil layers to reduce water loss in the aqueous zinc chloride cell.